Factors controlling the activity of HMG CoA reductase, the key regulatory enzyme of sterol biosynthesis, are being examined along the following lines of investigation: (a) The effect of alterations in membrane structure due to modification of the lipid composition of the membrane. (b) More effective methods of purification of the enzyme. (c) Studies on the mechanism of phosphorylation and dephosphorylation of the enzyme activity. (d) Production of antibody to the enzyme with the view of generating a simple immunoassay procedure to determine the variations in the level of enzyme protein in relation to changes in physiological regulation of HMG CoA reductase activity. (e) Detailed studies into the structure of active sites of the enzyme. (f) Regulation of the enzyme in intestinal mucosal epithelial cells, i.e., villi and crypt cells especially related to the interaction of bile salts, serum lipoproteins, and level of alimentation. (g) The turnover time of enzyme protein.